1. Field of the Invention
The present invention relates generally to a wireless power receiver, and more particularly, to a wireless power receiver that can maximize the transmission efficiency of wireless power.
2. Description of the Related Art
Mobile terminals such as, for example, a mobile phone or a Personal Digital Assistant (PDA), are driven by rechargeable batteries. The battery of the mobile terminal is charged through a separate charging apparatus. In general, a separate contact terminal is arranged outside of the charging apparatus and the battery, and the charging apparatus and the battery are electrically connected to each other through the separate contact terminal.
However, since the contact terminal is generally protrudes outwardly in a contact type charging scheme, the contact terminal is easily contaminated by foreign substances. Thus, battery charging is not correctly performed. Further, the battery charging may not be correctly performed when the contact terminal is exposed to moisture.
Recently, wireless charging or non-contact charging technology has been developed and used for electronic devices.
Wireless charging technology employs wireless power transmission/reception, and corresponds to, for example, a system in which a battery can be automatically charged if the battery is laid on a charging pad, without connecting the mobile phone to a separate charging connector. Wireless charging technology is generally known to be used with, for example, a wireless electric toothbrush or a wireless electric shaver. Accordingly, a waterproof function can be improved when electronic products are wirelessly charged through wireless charging technology. The portability of electronic devices can be increased since there is no need to provide a wired charging apparatus.
Wireless charging technology largely includes an electromagnetic induction scheme using a coil, a resonance scheme using a resonance, and a Radio Frequency (RF)/microwave radiation scheme converting electrical energy to a microwave and then transmitting the microwave.
A power transmission method through electromagnetic induction corresponds to a scheme of transmitting power between a first coil and a second coil. When a magnet approaches the coil, an induced current is generated. A transmission side generates a magnetic field by using the induced current, and a reception side generates energy through an induced current according to changes in the magnetic field. This phenomenon is referred to as magnetic induction, and the power transmission method using magnetic induction has a high energy transmission efficiency.
With respect to the resonance scheme, a system has been developed in which electricity is wirelessly transferred, using a power transmission principle of the resonance scheme based on a coupled mode theory, even when a device to be charged is separated from a charging device by several meters. A wireless charging system employs a concept that the resonance is a tendency of a wine glass to oscillate at the same frequency as a neighboring tuning fork. An electromagnetic wave containing electrical energy was resonated instead of sounds. The resonated electrical energy is directly transferred only when there is a device having a resonance frequency. Parts of electrical energy, which are not used, are reabsorbed into an electromagnetic field instead of being spread in the air, so that the electrical energy does not affect surrounding machines or people, unlike other electromagnetic waves.
The conventional wireless receiver changes impedance according to a charging amount, and accordingly the wireless charging efficiency is reduced. Therefore, technology is required that changes the impedance of the wireless power receiver based on the charging amount.